To answer your other questions:
I didn't understand what is gained by the "zener reference design" over the "Luxman" way either. But the Luxman design was described as "crude" in the build guide, so I guessed it must be somehow inferior.
You can follow the trace from the ground-end of D1, D2 and R4 (G2) all the way to the star ground (G1) so they're not at different potentials. I really can't guess why they are written at G1 and G2 on the schematic.

I guess that depends on your definition of 'inferior'. A zener reference will maintain a constant voltage reference which is the same regardless of the voltage on the input voltage rails (within certain limits) while the Luxman simple resistor design will give a constant voltage that is proportional to the rail voltage and so may require a little calculation based on the planned voltage rails. Additionally, if there is variation on the voltage rail for any reason (such as voltage sag at high current draw) then the reference voltage will vary proportionally. How much of an issue those drawbacks represent is a matter of personal judgement, and for most users in most circumstances the simple Luxman style voltage divider will be adequate. However, for the tiny price difference I went for the zener reference.

As noted by others, some devices had _very_ tight lead clearances, and the zener was one of these for me, if I recall correctly. I didn't have to drill out any holes, but I did have to force a few leads through with more force than I would have liked.

My understanding of the differing grounds is that they are effectively 'local' grounds, all interconnected via a star system. It is used both to restrict tiny variations in potential and to allow 'noise' a direct path to earth rather than flowing past multiple devices. This is the basic principle behind star grounding. Although in theory an earth is always 0.00V and is uniform, the reality is that this is not always the case and star grounding helps limit adverse effects and current flows. The short story is that Ostripper has done all the hard design work and we get to use the fruits of his labour.

Device selection was based on recommendations from Ostripper, component availability and price. Some items drift in and out of stock and so this will no doubt need some modification. Mouser stock a range of metal film resistors from TE Connectivity which I chose preferentially when possible as they are 600mW and a small form factor, but they didn't have stock in all values.

My selection preferences for electrolytic caps was long life and relatively low ESR, along with high voltage rating. This has resulted in a pair of relatively tall capacitors. I tried to use polypropylene rather than polyester caps for the audio signal path.

Note there there is no enameled wire, no heat sink compound and no Red LED.

I sourced the 0.22 ohm resistors from AnalogMetric. Lead spacing isn't exact but is easily widened.

R7 is a variable resistor. The build guide says to set it to between 70R and 100R, then does not say anything about adjusting it. So what is the purpose of it being adjustable? Why not just replace it with a 70R to 100R resistor? Does anyone know?

2.The small CCS just has a 20 turn 200R trimmer (R7). This CCS controls the LTP current. The small CSS can be adjusted between 1.8ma to almost 6ma, and this is achieved by adjusting R7 to between 70 and 100 ohms, which will result the desired output of 3.5ma at the "tail" of the LTP. 3.75mA gives roughly 8.25V across R14. Before powering on the amp, set R7 to 85ohms